South American Bird Partial Migration Project
From October 2004 to August 2008, a team of volunteers and biologists from the Museo de Historia Natural Noel Kempff Mercado and the University of Florida conducted research on the migratory behavior of birds at Caparú Ranch.
Because
highly seasonal conditions at the site, many birds
from thousands of herons and other water birds
to passerines such as flycatchers move in and out of
the area with the changing of the seasons.
South America is an interesting continent in which
to study the migratory behavior of birds because,
first, it is not well-studied, such that many ornithologists
are not even aware that a major migratory system exists
within the continent, one in which migrants breed
in the south-temperate latitudes of the continent
and move north towards tropical latitudes to escape
the austral winter. Secondly, South America is home
to an interesting form of migration in which not all
individuals of a population migrate, a condition termed “partial migration”.
Why would some individuals migrate hundreds of km while others,
members of the same population, not even move beyond the limits
of their 1 or 2 ha territory year-round? These extremes in behaviors,
exhibited even by members of the same population - which
presumably would have a similar genetic makeup - is an interesting
contradiction between what has been traditionally assumed to
be a behavior driven principally by genetic programming and
what is actually observed in nature. Apparently, because partial
migration is so common across the globe (though not as predominant
as in South America, where almost 70% of species may be partial
migrants), there is more complexity underlying the behavior
that a simple genetic on-off switch.
The basic question we are asking and trying to answer
at Caparú Ranch is: What behavioral, dietary and demographic
characteristics distinguish migrants from resident individuals
of the same population? To answer this question, we are studying
a population of Tropical Kingbird (Tyrannus m. melancholicus)
a member of the Tyrannidae, the most abundant bird
family in the Neotropics and the family contributing the
greatest number of migratory species to the continent’s migratory system.
We are postulating that partial migration is driven by the
individual condition of birds in a population such that first-year
individuals may migrate, due to their low social rank within
a dominance hierarchy of the population, which affects the quality
territories they can secure and as a consequence, their diet.
During the harsh dry season, such limitations may be enough
such that, for those first-year individuals, migrating away
after the breeding season before the dry season arrives, is
more profitable.
Methods
We are capturing and marking with color bands individuals
of the local Tropical Kingbird population, each individual
taking with it a unique color combination. We then conduct
surveys of the study site to find and observe marked individuals.
Over time, we should be able to document a pattern of presence/absence
of these marked individuals, if partial migration is operating.
Additionally, we plan on visiting the central Amazon Basin
near Manaus, Brazil to sample blood from Tropical Kingbirds
there, conduct population genetics analysis (e.g., utilizing
DNA microsatellites) and determine if individuals which disappear
from eastern Bolivia during the non-breeding season could be
migrating to the Amazon Basin to overwinter there. On preliminary
visits to Manaus as well as to Bolivia, ample evidence was produced
indicating that partial migration originating out of Bolivia,
with birds migrating into the Amazon Basin, is a strong possibility.
We are collecting 5 types of information on a marked population
of Tropical Kingbird in Bolivia: 1) demographics, 2) social
rank, 3) foraging success rate and food availability, 4)
population density, 5) nesting success, using the following
methods:
Demographics
Birds are captured (187 Tropical Kingbird individuals to date) using polyester mist nets (12X3.5 m, 38 mm mesh size) and individually color-banded (we are currently using 10 colors), as well as one aluminum, individually numbered band distributed by the Museo de Historia Natural Noel Kempff Mercado. Captured individuals are measured for all standard measures, including age (e.g., by skull ossification, juvenile plumage and primary shape), sex (e.g., by primary shape and brood patch), molt, wing chord, tail, culmen and tarsus length, fat class and weight. We also take blood (for population genetic analysis), feathers (for stable isotope analysis) and digital photos of wing, tail and body of each individual because some plumage characters can later be useful for sexing and ageing birds. For example, primary shape can be used to diffierentiate between hatch-year and after-hatch year individuals.
Social and Foraging Behavior
Color-banded individuals are searched for and observed daily utilizing binoculars or spotting scopes. After broad searches (>5 km) from the banding site, I have found that most banded individuals are located <1 km of the banding site. I have therefore divided the study site of approximately 100 ha into blocks of 300-400 m on a side. Within these, searches for individuals are conducted and, when a color-banded individual is encountered and bands identified, data is taken using a digital watch, noting start and end times. During this time, we take data on weather (i.e., wind, % cloud cover) and the following 3 areas of behavior:
- Social rank: To document social rank within the population, the number of aggressive encounters in which an individual participates in and wins or loses, and whether the individual was the initiator or receiver, will be counted. To do this, two observers will observe interactions, one observer observing one individual continuously. Kingbirds often show each other their crown patches during aggressive encounters. Crown patches may therefore serve as badges indicating social status or health and therefore as a way for rivals to “size each other up” before deciding whether to fight or not.
- Foraging success: To document foraging success rate, defined as the number of successful foraging maneuvers/time individual is observed, we have categorized 5 different foraging maneuvers, each of which is recorded when attempted by an observed individual. To compare these data to food resource availability between breeding and non-breeding seasons, we will sample aerial insect availability, on which Tropical Kingbird primarily feeds, between individual territories.
Nesting Success
During the breeding season (December - February), nests are searched for within blocks 300-400 m on a side and are monitored every 3-4 days. Nestlings are measured and color-banded following standard methods. Nesting success is defined here as successfully fledging at least one nestling out of one nesting attempt.
Index to Abundance
To permit an evaluation of whether relationships between age, sex, foraging success and migratory behavior operate on a density-dependent basis, the abundance of Tropical Kingbird individuals at the site will be taken every 2 weeks to serve as an estimate of population density. I will conduct censuses on a 1 km transect, 100 m wide, counting Tropical Kingbird, as well as all other bird species at the site using point count methodology during the breeding season and strip transect methods during the non-breeding season.
